The present invention relates to the fabrication of waveguide structures containing Bragg gratings.
Optical fibres have an optical waveguiding function defined by a high refractive index core surrounded by a lower refractive index cladding. Further functionality is available by writing grating structures, known as Bragg gratings, into the core. These structures comprise a series of grating planes of alternating high and low refractive index, and give a spectral reflection/transmission response which depends on the periodicity of the grating planes. These gratings can be used to create a wide range of fibre-based optical devices for applications including dispersion compensation, optical feedback and wavelength multiplexing.
Conventionally, fibre Bragg gratings are fabricated by exposing fibre to ultraviolet (UV) light so as to induce a refractive index modification in photosensitive material in the fibre core. This technique is dependent on the ability to create a predetermined pattern of exposed and unexposed regions of material, to give the desired periodic variation of refractive index. To achieve this periodicity, a common method is the use of an interference fringe pattern of UV light generated with a phase mask or two interfering beams. Exposure to bright fringes in the pattern causes a local increase in refractive index, whereas the dark fringes produce little or no change. Grating fabrication is typically performed by exposing a fibre from the side to the UV interference pattern, where the fibre has a waveguiding core doped with germanium to provide the required photosensitivity. The pattern is of a size much larger than the core width [1].
Phase masks are a well-established way of providing the required intensity pattern, which is formed as a large, elongate UV spot. This allows relatively long lengths of fibre to be exposed with an interference pattern containing hundreds of fringes, to give hundreds of grating planes. The pattern can be stepped along the fibre to give overlapping exposures so that each grating plane is exposed many times, which averages out any errors arising from instability of the writing system or imperfections in the mask. The physical definition of the mask limits the range of grating periods which can be achieved, but this can be improved by using wavelength detuning, in which the stepped exposures are achieved by pulsing at a period different from the period of the interference pattern This technique can also be used to create apodised and chirped gratings having a tailored spectral response.
An alternative method of generating a fringe pattern is to interfere two millimeter-scale LW beams by intersecting them at an appropriate angle. Alteration of the intersection angle changes the period of the fringes, so that Bragg gratings with a wide range of spectral responses can be readily produced. Also, the polarisation of the beams can be controlled to influence the contrast of the fringes, and the birefringence. Apodised and chirped gratings written using wavelength detuning can be produced from patterns of this kind.
Optical waveguiding can also be performed using planar waveguide structures. These are devices in which waveguiding is provided in one dimension by sandwiching a layer of high refractive index material between two lower refractive index layers. In the orthogonal dimension, a channel of high index material is defined from the layer by one of various channel writing techniques. These include reactive ion etching, ion exchange, and UV writing with a high intensity focussed spot in a photosensitive waveguiding layer.
The provision of Bragg gratings in such planar waveguide structures is desirable, because many of the diverse range of complex optical devices already available in an optical fibre format can thereby be densely integrated into a single robust waveguide chip. However, it has been found difficult to realise and optimise both the channel waveguiding core and the Bragg grating superstructure when defined in the same host substrate material.
The aforementioned techniques of writing gratings into fibres by UV exposure with an interference pattern have been applied to planar waveguide channels comprising photosensitive material [2, 3, 4]. However, the resulting devices are difficult and costly to produce. Also, the grating quality has been found to be poor and the spectral response of the grating is difficult to tailor, so applications of the resulting device are limited.
A particular problem arises from the need to have a very uniform effective refractive index along the length of the waveguiding channel, so as to give a grating with a non-varying reflection peak. However, the effective index of the guided mode depends strongly on the channel dimensions, which are difficult to control accurately when defining the channel by lithography and etching techniques.
If a UV channel writing technique is used, most of the photosensitivity of the waveguiding layer is used up in defining the refractive index difference between the channel and the remainder of the layer, so that there is little refractive index change available for creating sufficiently strong Bragg gratings via exposure to a UV interference pattern.
Hence, there is a need for an improved method of fabricating planar waveguides having Bragg gratings.